The proposed experiments will characterize gonadal steroid hormone concentrating cells in the rat brain by their neuroanatomical connections and by their content of neuroendocrinologically important peptides. Our previous steroid autoradiographic work demonstrated the exact anatomical location of the gonadal steroid concentrating cells in the hypothalamus and limbic structures and suggested their involvement in the control of neuroendocrine events, and the control of sexual behaviors. However, other than their position and number, information about these cells is sparse. Little is known about the specific neural connections or chemical content of these cells. (I) We will use the horseradish peroxidase (HRP) retrograde neuroanatomical tracing method to demonstrate the extrahypothalamic and intrahypothalamic connections of cell groups that contain gonadal steroid hormone concentrating cells. Using this information as a necessary and important guide, we will then (II) use the newly developed combination of either the fluorescent dye retrograde or the lectin retrograde neuroanatomical tracing methods steroid autoradiography to directly demonstrate the extrahypothalamic and intrahypothalamic connections of estradiol concentrating neurons. (III) We will also the newly developed combination of immunocytochemical methods with steroid autoradiography to directly demonstrate the luteinizing hormone-releasing hormone (LHRH) or oxytocin or antidiuretic hormone (HDH) content of estradiol concentrating cells. In order to understand how these hormone concentrating cells mediate the powerful effects of gonadal steroid on the brain, we must find out exactly where they fit into neural circuits regulating neuroendocrinological and behavioral events. This will lead to a better understanding of how gonadal steroids control these events. An important corollary of better understanding is a much improved chance at controlling these functions, particularly fertility.